In our U.S. Pat. No. 4,543,995, corresponding to the International patent application WO No. 84/02566 we have disclosed a plug coupling of the type definded before comprising a cylindrical housing, a piston shiftable within said housing and having formed at its front end clamping jaws extending out of said housing and radially displaceable via a ramp arrangement upon axial movement of the piston relative to the housing the movement being effected by selectively connecting a pressure source to one of various compartments formed between the inner surface of the housing and the piston by shifting a sleeve along the outer surface of the housing. Though such a plug coupling is easy and fast to handle and operates safely, the sleeve surrounding the housing has a relatively large outer diameter which makes it difficult to use the coupling in cases of closely arranged connections. Furthermore, providing the various radial bores is complicated and several sealings are required for sealing the various compartments against each other.
The U.S. Pat. No. 2,388,179 discloses a plug coupling using clamping jaws which may be radially widened, but are fixed to a tube. After inserting of a threaded connection the clamping jaws are pressed radially inward by a sleeve which is shiftable along the tapered outer surfaces of the clamping jaws. Such a plug coupling cannot be used one-handed. Furthermore, it is necessary to turn the coupling for getting a pressure-tight connection. According to a further aspect the clamping jaws of the plug coupling according to U.S. Pat. No. 4,543,995 are unitary with the piston which requires a relatively thin transition between the piston and clamping jaws which have to be resilient. Thus, there is a tendency of breakage of the clamping jaws at the transition point upon heavy use of the coupling. Furthermore, a certain force is necessary to overcome the resiliency of the clamping jaws when pressing them together.